How do I enable the Flip Active/Selected Bones options?
The operator properties are disabled when global undo is disabled



As for the menu variations, that would be to reflect the way the operator performs. Most of the select_mirror operators change the selection to be items on the opposite side, this can even lead to no active vertex in the selection. But the pose mode select_mirror operator will select the mirrored bone unless both bones are already selected when it will only change the active bone and not alter the selection.